154 Contribution to Knoivledge of Silver -Leaf Disease 



investigations of Percival, Pickering, Giissow, Brooks (cited above) 

 the basidiomycete Stereum purpureum is the cause of Silver-leaf disease 

 as was indicated by successful inoculation experiments. In all these 

 experiments the inoculation of the tree by Stereum was followed by the 

 external symptom of the silvering of the foliage, but the fungous hyphae 

 were never found in the leaves. I observed also that there is no trace 

 of any hyphae in the silvered Prunus domestica leaves. How are we 

 therefore to account for these remarkable phenomena of disorganisation 

 in the plum-leaf? If the mycelium of Stereum existed in the leaf, then 

 the changes described would be quite comprehensible. The diseased 

 leaf with its abnormal internal conditions would be classed without 

 any hesitation among the galls, as are the cases described by Reynolds. 

 It is true that a parasite can produce sometimes marked changes in 

 the nucleus even when it is not in immediate contact with the latter; 

 but in the case of Silver-leaf disease the mycelium of Stereum when 

 present is far distant. For the Stereum spreads — as it was established 

 by the authors mentioned — in the wood elements of root and stem but 

 never in the leaf nor its petiole. Since the attacked leaf behaves 

 exactly as a gall tissue parasitised by an organism living in the tissues, 

 it is difficult to believe a mycelium so remote could be the cause of such 

 changes of nuclei, chloroplasts and cytoplasm. 



Not only the changes of the protoplast, but also the spontaneous 

 maceration of the mesophyll tissue is difficult to explain as a result 

 of the action of a distant mycelium. The latter phenomenon, common 

 in the silvered leaves, reminds us somewhat of the results in Richter's 

 (after Kiister's Aufgaben, etc. 8, p. 462) experiments with narcotics 

 (vapours of camphor), where a similar maceration of tissue was obtained. 

 The falling asunder of cells, as it is known, may be caused in plants 

 parasitised by an organism in these ways; either the parasitic hyphae 

 penetrate and dissolve the cell walls by cytase or the host cells them- 

 selves produce the enzyme under the influence of the parasite. If 

 Stereum purpureum is the direct cause of the maceration of the leaves, 

 as has been suggested, then a fungous enzyme must be carried up by 

 the vascular bundles from the lower parts of the host. It is unlikely 

 that an enzyme of the nature of a cytase would be so carried since it 

 would cause the destruction of the vascular strands and other tissues 

 en route. It is much more probable that some toxin is secreted in 

 the leaves which causes the changes which have been described. The 

 action on the middle lamella of the lent' cells is probably due to an enzyme 

 secreted by the cells as result of poisoning, i.e. is an autolytic action. 



